1. Field of the Invention
This invention generally relates to surface acoustic wave devices and fabrication methods therefor, and more particularly, to a surface acoustic wave device having a seal resin portion with a cavity arranged in an upper portion of a surface acoustic wave element and a fabrication method therefor.
2. Description of the Related Art
Surface acoustic wave devices are for use in various circuits for signal processing in frequency range, for example, of 45 MHz to 2 GHz, such as transmit bandpass filters, receive bandpass filters, local oscillation filters, intermediate frequency filters, FM modulators, and the like. In recent years, the afore-mentioned signal processing devices have been increasingly small-sized, and there is a demand for reducing the sizes of electronic parts such as the surface acoustic wave devices used in the signal processing devices. In surface acoustic wave elements employed for the surface acoustic wave devices, comb-teeth electrodes (Interdigital Transducer: IDT) and reflectors are formed on a piezoelectric substrate with the use of metal films.
In portable electronics devices such as mobile telephones or the like, in particular, the electronic parts are increasingly used as a module. To this end, there is a need for the surface acoustic wave device that is low in height and that surface mount is available. At the same time, there is another need for providing a cavity on a top surface of electrode fingers of IDT, which are functional parts (oscillating part) of the surface acoustic wave device, so as to maintain the characteristics of the surface acoustic wave device. There is yet another need for ensuring hermetic sealing of the cavity to achieve the reliability of the surface acoustic wave element.
There has been proposed a method of forming a structure that includes a seal resin portion having a cavity in contact with the functional part of the surface acoustic wave element (namely, hollow structure), in order to satisfy the above-described needs. As disclosed in Japanese Patent No. 3,291,046 (conventional example 1), there is provided a method of forming a hollow structure by providing a melting resin in a region to be a cavity on the surface acoustic wave element, forming an upper plate on the melting resin, and removing the melting resin. As disclosed in Japanese Patent Application Publication No. 2003-523082 (conventional example 2), there is proposed a method of forming the hollow structure by forming a frame structure that surrounds an electronic structural element, applying an auxiliary film on the frame structure, providing a resin layer thereon, and removing portions other than a roof portion of the frame structure, so as to form a cavity on the electronic structural element. As disclosed in Japanese Patent No. 3,196,693 (conventional example 3), there is provided a method of forming the hollow structure by applying a resin film on a piezoelectric substrate on which multiple surface acoustic wave elements are formed, opening the resin film on the functional part of the substrate, and bonding the circuit board on the resin film. As disclosed in Japanese Patent No. 3,225,906 (conventional example 4), there is provided a method of forming the hollow structure by providing a light-sensitive resin on the substrate on which multiple surface acoustic wave elements are provided, opening the light-sensitive resin in an upper part of the functional part of the surface acoustic wave element, mounting an assembly of wirings and substrate, and separating by dicing. As disclosed in Japanese Patent Application Publication No. 2004-153412 (conventional example 5), there is provided a method of forming the hollow structure by forming a protection member that surrounds the surface acoustic wave element, and providing a protection film to cover the protection member and the functional part of the surface acoustic wave element.
In the hollow structures formed by the conventional examples 1 through 4, resins are provided on the piezoelectric substrate, demonstrating insufficient reliability. In the hollow structure formed by the conventional example 5, the protection film and the piezoelectric substrate are bonded by thermal compression or by adhesive, also demonstrating insufficient reliability.